Make a white color matte.
Insert the matte at the point where you want the flash, in a video track higher than the one containing your "main" video clip at that point.
Keyframe its opacity/transparency to have it quickly fade in and then quickly fade back out.

You could also use one of Vegas's built-in 'white flash' transistions. At the bottom left, click on the tab that says 'Transitions'. Drag the two clips on the timeline that you want to transition between together so that there's a crossfade between them, and then drag the white flash transition from the transition list onto the crossfade.

I'm a newb to so I'm not quite sure if this is how to do the effect I'm looking for. What I am trying to do is create a strobe light effect during a quick brief scene to synch up with a rythim fill riff in the song.

Is this how I do that? What is the easiest way to do that? Does vegas have a special effect already packaged in for this? Thanks everyone!